tvOS atmos auto detect not working
appletv id not auto detecting Dolby atmos and defaults to multichannel pcm.
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The Apple TV box outputs Dolby Atmos in Dolby MAT format, which some audio equipment may identify as Multichannel PCM. It is distinctly different from Dolby Atmos in Dolby Digital Plus format.
If your height speakers are working, then Dolby Atmos is working as intended.
If Apple TV can be set to use Dolby Atmos, then such audio tracks will be output as Dolby Atmos.
What is your brand and model audio system?
Try Top Gun: Maverick around 0:08:28 (test plane ‘Darkstar’ take-off) for a sweet overhead effect.